Double-focussing mass spectrometers which provide for angle and energy focussing are well known in the art. Such mass spectrometers may comprise electric and magnetic fields in tandem which are dimensioned so that the desired focussing conditions are obtained. However, the known double-focussing mass spectrometers use magnetic and electric deflection fields with relatively large deflection angles which result in a bent ion beam path between an entrance slit through which the ions to be analysed enter and an exit slit, through which ions with a selected charge/mass ratio pass to an ion detector. This makes the implementation by a compact and space-saving structure difficult.